Discrete time processing receivers are now common due to advance of high data rate sampling devices and digital signal processing. To measure the phase property of a Radio Frequency (RF) signal, the receiver should have much less phase noise than the RF signal itself. Usually, internal oscillation systems such as local oscillators and jitter of the internal sampling clock are the major causes of phase noise.
Some receivers have one or more frequency conversion stages. One example of a receiver is a Real Time Spectrum Analyzer (RTSA) available from Tektronix, Inc. of Beaverton, Oreg. In the RTSA, the phase noise of the local oscillator for the frequency converter is added to the received signal. Continuous signals are periodically sampled in discrete time processing receivers. The sampler is driven by an oscillator clock, which contains some timing jitter. The jitter is converted to additional phase noise by the sampler. Overall, the received signals are affected by many phase noise sources. Improving internal oscillators is a significant cost trade-off issue particularly with wide-range high-frequency synthesized oscillation systems. It is difficult, though, to produce oscillators having low noise for low cost.
Earlier attempts to reduce phase noise are shown in U.S. Pat. No. 7,746,058 to Nelson et al, and U.S. Pat. No. 6,564,160 Jungerman et al. Additional techniques are revealed in a published paper by Grove et al, entitled “Direct-Digital Phase-Noise Measurement” 2004 IEEE International Ultrasonics, Ferroelectrics and Frequency Control Joint 50th Anniversary Conference,” 2004.
The '058 patent describes a “Sequential Equivalent Time Sampling” system. The system includes variations on whether the trigger and the reference are synchronous with or are related to the DUT signal. In the disclosed methods, the reference signal must be digitized separately from the DUT signal, and includes complex method of converting the reference signal into “time stamps” that are used to correct the DUT samples. This is a complicated system.
The '160 patent uses a clock reference that is generated from or directly related directly to the DUT, or is provided by the DUT itself. The sampling of the DUT and reference are synchronous, and there is no mechanism for using a phase reference that is asynchronous to the DUT signal, which limits its application.
The Grove paper describes a “Phase Noise Test Set”, that can only measure the phase noise of an incoming signal. This has no ability to measure other parameters of a signal, to display a waveform, and not to be a receiver for demodulation signals.
Embodiments of the invention address these and other issues in the prior art.